The invention relates to forming ceramic coatings on substrates, and in particular to making bars or targets suitable for use as sources for forming such coatings by electron-beam physical vapor deposition.
A particular field of application of the invention lies in forming outer ceramic coatings on thermal barriers for metal parts made of superalloys, such as blades or nozzles in gas turbines.
In order to optimize the operation of gas turbines, in particular turbojets, it is desirable to make them operate at temperatures that are as high as possible, thereby increasing efficiency, reducing specific fuel consumption, and reducing polluting emissions (oxides of carbon, sulfur, nitrogen) and unburnt fuel.
The parts of turbojets that are exposed to the highest temperatures, and in particular the turbine nozzles and blades, are usually made of refractory metal alloys, or superalloys, based on nickel or cobalt, and they are provided with protective coatings.
Such protective coatings are generally multilayer coatings that form heat barriers and that are made up firstly of an outer coating made of a ceramic having low thermal conductivity, and secondly of an inner coating made of metal that protects the superalloy against oxidation and corrosion and that enhances bonding with the outer ceramic coating. The metal inner portion of the coating, or thermal barrier underlayer, is usually of the aluminide type. It may also be of the MCrAlY type where the metal M is Ni, Co, or Ni+Co. A film of alumina develops at the surface of the underlayer, thus enhancing bonding with the outer ceramic coating.
The outer ceramic coating is generally made by a electron-beam physical vapor deposition. Such a method enables a coating to be grown that has columnar morphology that is well suited to withstanding the thermomechanical stress differences in the various layers of the thermal barrier.
This deposition method, known by its abbreviation EBPVD, consists in introducing one or more substrates into a deposition chamber in which a source-forming bar or target has been placed of composition corresponding to that of the ceramic coating that is to be formed. In the particular intended application, the substrates are metal parts made of superalloy and already provided with a metal underlayer of a thermal barrier.
The bar is placed vertically and its top surface is swept by an electron beam which leads to surface melting of the bar material. The chamber is under a vacuum, thus allowing a cloud to form coming from evaporation of the bar material. The evaporated material deposits by condensing on the substrates which are caused to rotate in front of the evaporation cloud. The process is continued until the desired thickness has been obtained for the coating, with the bar being consumed progressively.
The material most commonly used for the ceramic layers of thermal barrier coatings for gas turbine parts made of superalloy is zirconia (ZrO2) stabilized by yttrium oxide (Y2O3) known as “yttrium-stabilized zirconia” or “YSZ”.
Nowadays, and even more in the future, the performance required of these ceramic coatings is such that they can no longer be made as a coating constituted by one layer of a single ceramic material.
That is why proposals have been made to make multilayer ceramic coatings with layers of different compositions that perform different functions.
Thus, U.S. Pat. No. 5,683,825 describes a ceramic coating with a layer of erosion-resistant material made of alumina or silicon carbide deposited by physical vapor deposition on a layer of columnar YSZ.
Patent application WO 00/09778 describes a ceramic coating having a top layer made of zirconia stabilized with hafnium or scandium oxide possessing high thermal stability at high temperatures, and a lower layer of YSZ providing bonding with the metal underlayer.
Once the provision of multilayer ceramic coatings requires a plurality of deposition cycles to be performed using different sources, costs become very high.
Proposals have been made to provide ceramic coatings in a continuous stage, but at the price of making the physical deposition method more complex.
Thus, European patent application EP 0 705 912 proposes reducing thermal conductivity by alternating electron-beam physical vapor deposition (EBPVD) with plasma-assisted physical vapor deposition (PAPVD).
In application WO 96/11288, thermal conductivity is reduced by alternating deposits of nanometer layers of YSZ and of alumina, the deposits being formed by EBPVD using two distinct sources.
U.S. Pat. No. 5,350,599 proposes making a multilayer structure by modifying the morphology of consecutive layers of the same composition by varying the rotation of the substrates to be coated.
As for U.S. Pat. No. 5,792,521, it proposes making multilayer deposits by scanning different sources formed by targets positioned in a special manner in the deposition chamber.
Proposals have also been made to make a protective coating that presents a composition gradient within its thickness by using EBPVD and a bar or source that contains the various components for the layers of the coating.
Thus, U.S. Pat. No. 6,287,644 describes a process during which the various components of the bar are evaporated in succession in order of decreasing vapor pressure. A protective coating is thus formed whose composition varies continuously from a metal underlayer deposited on a superalloy part to an outer ceramic coating.
Patent application EP 1 096 037 describes using a bar constituted by a composite ingot made up of a block of YSZ having inserts included therein, the inserts being made of metal, or of a metal-ceramic mixture, or of ceramic, each insert occupying a fraction of the cross-section of the ingot.
In the last two methods mentioned above, the surface scanned by the electron beam always contains materials for different layers of the coating that is to be formed. In spite of having different vapor pressures, it is not possible in practice to control the method in such a manner as to obtain a precise predetermined composition in each layer of the resulting coating.
In addition, it is difficult to make an ingot having inserts of different shapes as in EP 1 096 037.
A physical vapor deposition method using an ingot made up of non-sintered ceramic powders of different grain sizes is described in U.S. Pat. No. 6,168,833.
The possibility of varying the composition of the ingot in order to make a coating with a composition gradient is mentioned but without further details.
Finally, European patent EP 1 158 061 discloses a method of forming a stratified material made up of powder layers having different compositions which are compressed in a mold and sintered. The material is used for making a cylindrical or rectangular bar by cutting through the superposed layers of material. The bar is subjected to heat treatment so as to form, by melting, a fiber having a composition gradient.